mm/memory_hotplug: enforce section granularity when onlining/offlining

Already two people (including me) tried to offline subsections, because
the function looks like it can deal with it.  But we really can only
online/offline full sections that are properly aligned (e.g., we can only
mark full sections online/offline via SECTION_IS_ONLINE).

Add a simple safety net to document the restriction now.  Current users
(core and powernv/memtrace) respect these restrictions.

Signed-off-by: David Hildenbrand <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Reviewed-by: Oscar Salvador <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Wei Yang <[email protected]>
Cc: Baoquan He <[email protected]>
Cc: Pankaj Gupta <[email protected]>
Cc: Charan Teja Reddy <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Fenghua Yu <[email protected]>
Cc: Logan Gunthorpe <[email protected]>
Cc: "Matthew Wilcox (Oracle)" <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Michel Lespinasse <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Tony Luck <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

mm/memory_hotplug.c

@@ -809,6 +809,11 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages,
 	int ret;
 	struct memory_notify arg;
 
+	/* We can only online full sections (e.g., SECTION_IS_ONLINE) */
+	if (WARN_ON_ONCE(!nr_pages ||
+			 !IS_ALIGNED(pfn | nr_pages, PAGES_PER_SECTION)))
+		return -EINVAL;
+
 	mem_hotplug_begin();
 
 	/* associate pfn range with the zone */
@@ -1494,6 +1499,11 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages)
 	struct memory_notify arg;
 	char *reason;
 
+	/* We can only offline full sections (e.g., SECTION_IS_ONLINE) */
+	if (WARN_ON_ONCE(!nr_pages ||
+			 !IS_ALIGNED(start_pfn | nr_pages, PAGES_PER_SECTION)))
+		return -EINVAL;
+
 	mem_hotplug_begin();
 
 	/*